Welding methods and welding apparatuses are continuously being developed for achieving higher deposition rates at maintained or improved weld quality. One way of increasing deposition rates is to use a plurality of electrodes operating in a single welding process in a single weld puddle. Use of a plurality of welding electrodes in a single weld puddle requires that the electrodes are located in a close proximity to each other. Close arrangement of electrodes will however lead to interaction between magnetic fields generated by each electrode. This may result in distorted welds.
Two different approaches for using a plurality of welding electrodes in a single welding process are known, tandem and twin welding.
In tandem welding each electrode is powered via a separate power source. Tandem welding may be advantageous in many applications since the control of power supply to each electrode is facilitated. In tandem welding, the magnetic interference between electrodes may be reduced by supplying phase shifted welding currents to the electrodes. This may be accomplished by conventional Scott-coupled power sources or by use of complex power sources, such as high frequency converters, for instance.
Since tandem welding requires the presence of two separate power sources, tandem welding apparatuses tend to be expensive and bulky. Hence, for some applications, such as welding in constrained spaces, twin welding may be a preferred or only possible solution.
In contrast, in twin welding a single power source is used for feeding a welding current to the electrodes. An advantage of twin welding methods is that the deposition rate may be increased for a given amount of power input. In twin welding methods, the use of more than one electrode allows for reduction in electrode diameter, which in turn increases the current density for each electrode. The increased current density allows for increased pre-heating of the electrode, hence a higher deposition rate can be maintained at less heat transfer to the weld puddle.
In early twin machines, such as for example disclosed in DE2107617, twin welding processes were enabled by arranging the feed current rectification to be applied such that one arc is supplied with positive and the other by the negative half wave current. Such arrangements led to severe interaction between the electrodes. In order to reduce the problem with interaction, the electrodes had to be positioned at a relatively large distance from each other, making it difficult to allow both electrodes to operate in the same weld puddle. If separation of the welding processes for each electrode takes place the weld process will be deteriorated.
In order to allow for closer positioning of the electrodes to ensure that both electrodes may operate in the same weld puddle, it has been suggested to supply twin wires through a common an electrically conductive contact element which ensures that both wires assumes the same potential. An example of this type of twin wire welding arrangement is disclosed in U.S. Pat. No. 5,155,330.
Even though twin welding methods, where consumable electrodes are fed through a common electrically conductive contact element to ensure that both wires assumes the same potential, has shown to be beneficial for both gas metal arc welding methods and submerged arc welding methods, further improvements are desirable in order to improve weld quality as well as to increase the deposition rate in the welding process.